Among two-dimensional (2D) materials that recently have attracted great interest, black phosphorus (BP) is gaining a rising popularity owing to its unique structure and intriguing anisotropic properties, which are rarely found in other 2D materials. Apart from its anisotropic structure, BP shows a layer dependent bandgap, which can be tuned from 0.3 eV to 2 eV with decreasing thickness from bulk BP to single-layer BP, which is also known as phosphorene.
BP can be prepared from white or red phosphorous at high temperature and under high pressure. BP crystal is composed of puckered-honeycomb layers stacked by weak van der Waals interactions, giving the possibility of preparing phosphorene via different exfoliation methods. A number of top-down methods such as sonication-assisted liquid exfoliations in different solvents, plasma-assisted treatment, mechanical approaches, and bottom-up synthetic methods can be used for the exfoliation of BP.
In recent years, bipolar electrochemistry has grown extensively in applications such as exfoliations of bulk graphite to single-layer two-dimensional graphene. Unlike a conventional two-electrode or three-electrode electrochemical cell in which each electrode is directly connected to a potential source, a bipolar cell has a bipolar electrode placed in a solvent between two driving electrodes. Despite that, the bipolar electrode itself is not electrically connected to the power supply, and the electric field between the driving electrodes drives reactions on the two ends of the bipolar electrode such that one end of the bipolar electrode can act as the anodic pole where molecules are oxidized on the electrode and the other end can act as the cathodic pole where molecules are reduced and the redox reactions occur in the bath.